This invention relates generally to the storage and subsequent recovery of energy and more specifically to the compression and storage of air during a compression mode in a Compressed Air Energy Storage (CAES) system and the subsequent recovery of such stored energy during a power generation mode in a turboexpander train wherein compressed air input to a high pressure turbine is reheated in a recuperator without the need for high pressure combustor(s), and high pressure turbine exhaust is heated in a low pressure combustor.
CAES power plants have become effective contributors to a utility's generation mix as a source of peaking or intermediate energy and spinning reserve. CAES plants store off-peak energy from relatively inexpensive energy sources such as coal and nuclear baseload plants by compressing air into storage devices such as underground caverns or reservoirs. Underground storage can be developed in hard rock, bedded salt, salt dome or aquifer media.
Following off-peak storage, the air is subsequently withdrawn from storage, heated, combined with fuel in combustors and expanded through expanders, i.e., turbines, to provide needed peaking/intermediate power. Since inexpensive off-peak energy is used to compress the air, the need for premium fuels, such as natural gas and imported oil, is reduced by as much as about two thirds compared with conventional gas turbines.
Compressors and turbines in CAES plants are each connected to a synchronous electrical machine such as a generator/motor device through respective clutches, permitting operation either solely of the compressors or solely of the turbines during appropriate selected time periods. During off-peak periods (i.e., nights and weekends), the compressor train is driven through its clutch by the generator/motor. In this scheme, the generator/motor functions as a motor, drawing power from a power grid. The compressed air is then cooled and delivered to underground storage.
During peak/intermediate periods, with the turbine clutch engaged, air is withdrawn from storage and then heated and expanded through a turbine to provide power by driving the generator/motor. In this scheme, the generator/motor functions as a generator, providing power to a power grid. To improve the CAES heat rate, waste heat from a low pressure turbine exhaust is used to pre-heat high pressure turbine inlet air in a recuperator. The compression process of the CAES plant is characterized by a much higher overall compression ratio than that for conventional gas turbines. This requires multistage compression with intercoolers in order to improve CAES plant efficiency.
For a more complete discussion of CAES systems, see Nakhamkin, M. et al. "Compressed Air Energy Storage: Plant Integration, Turbomachinery Development", ASME International Gas Turbine Symposium and Exhibition, Beijing, Peoples'Republic of China, 1985 and Nakhamkin, M. et al. "Compressed Air Energy Storage (CAES): Overview, Performance and Cost Data for 25MW to 220MW Plants", Joint Power Generation Conference, Toronto, Canada, 1984, both incorporated herein by reference.
The turbomachinery associated with a conventional CAES plant has high pressure and low pressure turbines with high pressure and low pressure combustors, respectively. Fuel is mixed with compressed air and combusted at essentially constant pressure in these combustors, thus producing mixtures of products of combustion with high temperatures. The high temperature mixtures are then expanded in series through the high pressure and low pressure turbines, thereby performing work. Each turbine generally has an optimum expansion ratio (i.e., ratio of turbine input pressure to turbine output pressure) resulting in the highest possible efficiency for a specific turbine inlet temperature. The efficiency and optimum pressure ratio increase with increasing turbine inlet temperatures.
Turbine trains used in CAES systems have associated therewith an overall expansion ratio which is the product of expansion ratios of individual turbines which are serially connected. The overall expansion ratio of a turbine train comprising high and low pressure turbines is the ratio of turbine train input pressure (to a high pressure turbine) to turbine train output pressure (exhaust from a low pressure turbine), and generally ranges for CAES applications from 20to 100 or more.
The overall expansion ratio is distributed between the high pressure and low pressure turbines in order to result in the highest possible efficiency. For equal high pressure and low pressure turbine inlet temperatures, the expansion ratios of the high pressure and low pressure turbines are approximately equal for a conventional CAES plant having a recuperator. For example, for the illustrative case in which the overall expansion ratio is 60, the expansion ratio of the high pressure and low pressure turbines will be .sqroot.60.
The expansion ratio of a single turbine may be modified by the addition or deletion of component turbine stages. An increase in the number of component turbine stages in a turbine generally relates to an increase in the expansion ratio of the turbine. Typically, turbines having an expansion ratio of four have two turbine stages, turbines having an expansion ratio of .sqroot.60 have three turbine stages and turbines having an expansion ratio of fifteen have four or five turbine stages.
Due to generally high air storage pressures, CAES plants are associated with high operating pressures and high expansion ratios. A proposed solution is to design a high pressure combustor which operates at pressures associated with CAES plants. However, there is no operating experience with combustors at the pressures that are encountered in high pressure turbines of CAES plants, although combustors operating at pressures up to 30 bar are relatively numerous and are adequate for use with low pressure turbines. More specifically, there is no operating experience with the high pressures of 60 bar and above that may be encountered in the high pressure turbines of CAES systems.
An additional proposed solution is to throttle the pressure of the compressed air input of the expansion train to a pressure level at which combustors are presently available. However, such a solution is not economical due to the waste of energy, stored as pressure in the compressed air, in reducing the pressure level prior to input to the expansion train.
Since adequate high pressure combustors are not believed to exist, nor is system operation at a reduced pressure economical, a need arises for a CAES system which does not require a high pressure combustor for proper operation.